Seal arrangements for lamps

ABSTRACT

Lead-in wires of lamps are disposed in a vitreous silica envelope through the use of a graded seal in which two abutting segments of refractory metal wires are coated with a cermet formed of an admixture of powdered refractory metal and vitreous silica. These lead-in wires are disposed in an aperture in a wafer which, in turn, is sealed to a vitreous silica envelope.

United States Patent Loughridge et al.

[ 51 July 11, 1972 [54] SEAL ARRANGEMENTS FOR LAMPS [72] lnventors: Frederick A. Loughridge, Manchester;

Wilfrid G. Matheson, Marblehead; Edmund M. Passmore, Wilmington, all of Mass.

[73] Assignee: Sylvania Electric Products Inc.

[22] Filed: Dec. 29, 1969 21 Appl. No.: 888,299

2,556,059 6/1951 Braunsdorff ..316/19 X 2,617,068 11/1952 Spinnler et al.. .....l74/50.6l X 2,791,480 5/1957 Larson ..65/59 X 3,132,279 5/1964 Lewin ..313/318 X 3,203,084 8/ I965 Best ..65/59 UX 3,363,133 H1968 Harris et al.. .....313/2l7 3,363,134 1/1968 Johnson ..313/220 3,405,303 l0/l968 Koury et al. ..313/217 3,139,553 6/1964 Schwartzwalder.. ..313/144 3,348,091 lO/l967 Abdella ..313/145 X 3,497,752 2/1970 Peterson .l.....3 1 3/222 X Primary Examiner-Roy Lake Assistant Examiner-Palmer C. Demeo Attorney-Norman J. O'Malley and Owen J. Meegan [57 ABSTRACT Lead-in wires of lamps are disposed in a vitreous silica envelope through the use of a graded seal in which two abutting segments of refractory metal wires are coated with a cermet formed of an admixture of powdered refractory metal and vitreous silica. These lead-in wires are disposed in an aperture in a wafer which, in turn, is sealed to a vitreous silica envelope.

7 Clains, 3 Drawing figures P'ATEN'TEDJUL 1 1 I972 FREDERICK A. LOUGHRIDGE WILFRID G. MATHESON EDMUND M. PASSMORE INVENTORS QQQL a ATT R'NE BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates, in general, to electric lamps utilizing an envelope of vitreous silica, containing an electrical energy translating means and which are adapted to operate at high temperatures and/or pressures.

2. Description of the Prior Art In making a hermetic seal between a lead-in wire or conductor and a vitreous envelope in the prior art, it has been necessary either that the conductor have the same coefficient of expansion as the glass into which it is sealed, or that very thin foil sections of metal be used to compensate for the unequal coefficients of expansion. The use of materials having similar rates of expansion is generally followed in the manufacture of incandescent and low pressure discharge lamps. Both of these types of lamps operate at relatively low temperatures. However, where the lamp operates at very high temperatures and particularly where the lamp envelope is made of quartz, fused silica or quartz-like high temperature glasses, the use of such techniques is inapplicable. The reason for the inapplicability is that the high fabrication temperatures necessitated by the very high fusing temperature of quartz restricts the choice of metals for the lead-in wires to only a few, such as tungsten, tantalum or molybdenum. Only these metals can withstand the high temperatures. However, the coefficient of thermal expansion of either of these three metals is very much greater than that of quartz, as much as times greater. Accordingly, recourse has generally been to the second technique involving the use of the foil or ribbon seals.

In a quartz envelope, a hermetic seal can be formed with either tungsten, tantalum, or molybdenum sheets or ribbons, provided that the metal is sufficiently thin. Molybdenum is generally used because of its ductility and the fact that it is readily available. Owing to the much greater coefficient of expansion of molybdenum as compared with that of quartz, stresses will be set up in the seal at different temperatures. Provided the metal is very thin and properly bonded to the quartz, such stresses may be reduced to the point where they will not produce any cracks. In other words, the metal having become bonded to the quartz at a relatively high temperature will merely go into tension when the quartz cools but will not rupture nor crack the seal.

Such molybdenum ribbon seals have generally been produced by welding thin strips of foil or ribbon to suitable lengths of wire. The molybdenum foil is cut into strips a few millimeters wide and of the proper length required for the seal, and thicker molybdenum wires are welded to each end of a strip to form the current connection. The ribbon or foil is preferably etched in an electrolytic bath which cleans the surface thoroughly and roughens it slightly. The bath also dissolves away some of the metal so that feather edges which are produced assist in preventing leakage around the seal.

Lamps with this type of connection and seal are expensive to fabricate because of the several welding steps which are required. Moreover, each weld introduces the additional possibility of embrittlement of the foil and the subsequent failure of the lamp.

A different approach to the same problem is disclosed in the United States patent to Noel, US Pat. No. 2,667,595. Therein, the lead wires are made of a highly refractory metal such as molybdenum, tungsten or tantalum and consist of a single piece of wire whereof an intermediate portion is foliated by longitudinal elongation so as to provide a gradual change in the shape of the cross section. Such a lead wire may be made, for instance, by rolling a piece of molybdenum wire longitudinally between a pair of hardened cylindrical rollers. Although seals using this technique perform satisfactorily, rolling very short lengths of refractory metal wire is rather difficult. Moreover, while uniformity in thickness of the foliated portion is important, it tends to be rather difficult to obtain on a mass production basis.

SUMMARY OF THE INVENTION According to the present invention, two refractory metal wires are abutted against each other, end to end, so that a good electrical connection is made. Preferably, the two wires are butt-welded to insure the connection. A cermet-forming mixture of a powdered refractory metal and ground quartz, both having a particle size of less than about 200 mesh, is made into a paste with an organic vehicle and an organic binder. The paste is coated around the abutment and a short distance on either side thereof. Following the application of the paste, the coated-and welded wire is placed into an aperture in a pre-formed base and heated in a vacuum to simultaneously consolidate the paste into a dense mass and bond the wires to the sides of the aperture. Alternatively, a single wire may be used with the paste describedabove disposed about the middle.

The pre-formed base is in the shape of a wafer and formed of a high melting glass such as one having a coefficient of expansion between about 5 X 10' to 40 X 10" cm/cm/C and a working temperature between about 1,000C and l,700C. Materials such as calcium alumino-silicate glasses and formed silica are particularly desirable for use with quartz envelopes.

When the base unit has been assembled, the wafer and the ends of the quartz tube which fonn the envelope are joined together using conventional sealing techniques. If the quartz tube is open at both ends, the operation of sealing the second wafer to the tube can be repeated or done simultaneously with the sealing of the first wafer. In some instances, it is necessary to place more than one lead-in wire in a single wafer. In those instances, two wires are placed in two apertures. Then, the graded seal is formed in the manner disclosed above.

Preferably, the apertures are formed in a conical shape with the widest portion of the aperture opening inwardly, with respect to the envelope. The narrowest portion of the aperture is only slightly wider than the outside diameter of the wire which fits into it, thereby providing a relatively tight fit/When the cermet paste is heated, it flows into the rest of the aperture and makes a rugged bond.

In order to provide a good seal between the lead-in wire and the wafer, we have found that the composition should be between 45 and weight percent quartz and the balance powdered refractory metal. Below the limit, there is insufficient quartz to form a cermet which will adhere to the wafer. Above this limit, the thermal expansion coefficient is so low that it will not prevent breaking of the lamp when operated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a pair of butted lead-in wires with the cermet disposed about the abutment.

FIG. 2 is a side view of the two lead-in wires before they are disposed in the wafer and before the wafer is sealed to the envelope.

FIG. 3 is a side view of the assembled lamp with the lead-in wires sealed in the wafer which, in turn is sealed to the envelope.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, the lead-in wire includes an outer segment 1 which is in electrical-conducting relationship with inner section 2. Preferably, the two ends of the segments are butt-welded to each other. This operation includes butting together two of the ends of the segments which are to be welded. A current is passed through the segments and they are I A layer of cermet-forming material 3 is placed around the joint formed by butt-welding the two segments. The cennet forming material is preferably prepared by mixing finely divided refractory metal and quartz, generally in amounts between about 45 and 85 weight percent quartz. These materials are generally less than about 200 mesh. The powders are mixed with a vehicle and a binder, such as acetone and paraffin respectively, to form a slurry of paint-like consistency.

When prepared, the slurry is painted around the joint between the two segments with a fine brush. Sufficient slurry is used to cover the joint and make a small drop, generally covering about one-eighth inch of the segments on either side of the joint.

Referring to FIG. 2, to make the lamp, the lead-in wire described above is fitted into holes or apertures which have been formed in the wafer 4, previously mentioned. The apertures are conical or tapered in shape so as to provide an opening 5 which is wider in diameter on one side of the wafer than the other. The width of the smaller-diameter portion of the aperture is just slightly greater than the outside diameter of the wire so as to provide a relatively snug fit. The outer segment 1 of the wire is then placed into the aperture 5 first through the wider diameter portion and then into the narrower diameter portion. Paste 3, and hence the joint which it covers, is forced into the wider diameter portion of the aperture 5. Generally, the joint between the inner and outer segments of the lead-in wire should be within the confines of the aperture 5.

The paste of refractory metal and quartz are then simultaneously consolidated into dense masses in a vacuum and bonded to both the lead-in wire and the base by attaching the ends of the segments to opposite sides of a power supply. When the wire is heated, the consolidation and bonding take place. Care should be exercised to avoid heating the wires to a point where there would be a change in the crystal structure.

Of course,-more than one lead-in wire can be disposed in more than one aperture, as is shown in the drawings. Some lamps, for example single ended quartz halogen lamps or high pressure electric discharge devices having starting probes, require two lead-in wires on one side of the envelope.

When the wafer 4 and lead-in wire are firmly joined together, the assembly is ready for mounting upon the end of a quartz tube or envelope 6 as shown in FIG. 3. Such sealing is accomplished utilizing conventional techniques by heating the wafer 4 and the tube 6 to the softening range. The wafer 4 and the tube 6 seal to each other and are ready for subsequent steps of conventional lamp manufacturing.

Many modifications of the present invention can be made. For example, as indicated above, we prefer to paint a cermetforming paste upon the joint and form the cermet by heating the wires. This may be accomplished also by radiant heating of the assembly to a temperature which effectuates the bond. Moreover, it may sometimes be desirable, because of the difference in coefficients of expansion between the lead-in wire and the quartz to paint several coats of the cermet upon the wire. Each coating will have a coefficient which is less than the one beneath it so that a good seal can be formed. Such variations in the coefficient can be attained by varying the ratio of powdered refractory metal to powdered quartz, that is including the quartz content for each coating.

As our invention, we claim:

1. A lamp comprising: a vitreous silica envelope; a seal at one end of said envelope, said seal comprising a vitreous silica cylindrical wafer disposed within said end of said envelope and peripherally sealed thereto; a refractory metal outer leadin wire segment disposed outside of said envelope and extending into said wafer; a refractory metal inner lead-in wire segment disposed inside said envelope and extending into said wafer; said inner segment and said outer segment being in end to end abutting and electrical conducting relationship with each other; a coating disposed in said wafer around the point of abutment of said lead-in wires, said coating being formed of a mixture containing about 45 to weight percent of powdered vitreous silica and the balance powdered refractory metal; said coating providing a seal between said lead-in wires and said wafer; and light generating means within said envelope electrically associated with said inner lead-in wire.

2. The lamp according to claim 1 wherein the lead-in wires are formed of tungsten or molybdenum.

3. The lamp according to claim 1 wherein said wafer has an aperture disposed therein for the location of said abutment of said lead-in wires.

4. The lamp according to claim 3 wherein the aperture is conical in shape, the widest portion of the aperture facing into the interior of the envelope.

5. The lamp according to claim 4 wherein the inner lead-in wire is tungsten and the outer lead-in wire is molybdenum and the powdered refractory metal is tungsten.

6. The lamp according to claim 4 wherein the inner lead-in wire is welded to the outer lead-in wire.

7. The lamp according to claim 5 wherein the powdered refractory metal has a particle size of less than about 200 mesh. 

2. The lamp according to claim 1 wherein the lead-in wires are formed of tungsten or molybdenum.
 3. The lamp according to claim 1 wherein said wafer has an aperture disposed therein for the location of said abutment of said lead-in wires.
 4. The lamp according to claim 3 wherein the aperture is conical in shape, the widest portion of the aperture facing into the interior of the envelope.
 5. The lamp according to claim 4 wherein the inner lead-in wire is tungsten and the outer lead-in wire is molybdenum and the powdered refractory metal is tungsten.
 6. The lamp according to claim 4 wherein the inner lead-in wire is welded to the outer lead-in wire.
 7. The lamp according to claim 5 wherein the powdered refractory metal has a particle size of less than about 200 mesh. 